Processes for isomerization of paraffin components in naphtha to give gasoline blending components with high octane numbers require a catalyst with high activity and stability to take advantage of the relatively high equilibrium conversions at low temperature.
In the case of n-pentane isomerization, although neopentane is thermodynamically possible the only product observed is iso-pentane. Neopentane is not produced as a major product during isomerization, perhaps as a result of kinetic factor.
For n-hexane isomerization the yields of 2,3-dimethylbutane and the sum of the yields of 2-methylpentane and 3-methylpentane are relatively temperature independent. Therefore, when describing the activity of a naphtha isomerization catalyst a measure of activity for n-hexane isomerization is its ability to convert n-hexane to 2,2-dimethylbutane at a given temperature.
The octane numbers of the equilibrium fractions of both C.sub.5, C.sub.6 and mixtures of C.sub.5 /C.sub.6 tend to increase linearly with decreasing temperature (FIG. 1). Given the fact that the research octane numbers of n-hexane and n-pentane are .about.25 and .about.62 respectively, substantial octane increases can be obtained via isomerization. The octane of a light naphtha can be increased by 10-12 RON (Research Octane Numbers) through single-pass operation (FIG. 1) and if the normal paraffin are separated from the isomerized product and recycled for further reaction an octane increase of 20 RON can be realized. In addition, the isomerization process has other advantages, e.g., it is a low temperature (200.degree.-300.degree. C.), low pressure (&lt;500 psi) process.
Examples of zeolites used as light naphtha isomerization catalysts include faujasite (Zeolite Y), U.S. Pat. No. 3,236,904, and mordenite, U.S. Pat. No. 3,190,939.
Several studies comparing the activity and properties of mordenite and faujasite (e.g., zeolite Y) catalyst have been carried out. Gray and Cobb, J. Catal., 36, 126 (1975) state that the active sites of mordenite are similar to their counterparts in faujasite. Despite similarities each catalyst has its own unique properties. For example, steric effects are more pronounced in mordenite since it has a slightly smaller pore size than zeolite Y (faujasite) and does not contain the large macropores of the faujasites (Yashima and Hara, J. Catal., 27, 329 (1972)). Further, mordenite has an unidimensional pore system, whereas the faujasites have three dimensional pore systems.